import glob
import json
import os


def get_ordered_files(dir_name):
    # Get list of all files only in the given directory
    list_of_files = filter(os.path.isfile, glob.glob(dir_name + '/*'))

    # Sort list of files based on name
    list_of_files = sorted(list_of_files)

    return list_of_files

def get_next_tn(dir_name):
    current_path = dir_name +"/current.json"
    tns = get_ordered_files(dir_name)
    
    if os.path.isfile(current_path):
        with open(current_path, 'r') as openfile:
            current_tn = json.load(openfile)

        tn_name = ""
        for i, tn in enumerate(tns):
            if tn == current_tn["current"]:
                tn_name = tns[(i+1)%len(tns)]
                break

        if tn_name == current_tn["current"]:
            # nothing changed
            return None
        elif tn_name == "":
            # old thumbnail was not found in dir
            tn_name = tns[0]
    else:
        tn_name = tns[0]
    
    current_tn = {"current":tn_name}

    with open(current_path, "w") as outfile:
        outfile.write(json.dumps(current_tn))

    return tn_name